1. Field of the Invention
The present invention relates to the use of pulverulent thermosetting coating compositions based on physical mixtures of individual isocyanato isocyanurates formed from aliphatic and (cyclo)aliphatic and/or cycloaliphatic diisocyanates and from polyesters containing hydroxyl groups and terephthalic acid for coil powder coating materials, to a process for producing such coatings, and to the metal coils coated with such coating materials.
2. Description of the Background
Thermosetting powder coating compositions are used intensively for producing crosslinked coatings on a wide variety of substrates. Thermosetting coatings are generally harder than thermoplastic compositions, are more resistant to solvents and detergents, possess better adhesion to metallic substrates, and do not soften on exposure to increased temperatures.
Since 1970, thermosetting pulverulent materials have been known which are obtained by reacting a hydroxyl-containing resin with a blocked polyisocyanate. Of the blocked polyisocyanates, isophorone diisocyanate adducts blocked with xcex5-caprolactam have become established as curatives for PU powders. The PU powders prepared using these curatives are employed for coating a wide variety of articles made of metal, on account of their superior weathering stability and thermal color stability. Powders of this kind are described, for example, in DE 27 35 497. Using these powders, ready-formed metal components are coated piece by piece (post-coated metal).
Coil coating, on the other hand, is a process for coating metal coils at speeds from 60 to 200 m/min. Metal sheets, preferably of steel or aluminum, are cleaned and coated with a paint. These metal sheets are then passed on for further processing, where they acquire their actual form. The principal applications are trapezoidal profiles coated with weather-resistant paints, for roofs and facings of buildings, for example, and also doors, window frames, gates, guttering, and blinds. For the interior, coil-coated metal sheets are employed primarily for partition walls and ceiling elements. Other fields of use include steel furniture, shelving, shopfitting, and appliance casings. Lamps and light fittings form a further important application segment. There is also a broad applications pallet in the vehicle segment. Truck bodies and exterior-mounted automotive components are often manufactured from precoated materials.
For coating the substrate used, a pretreatment is generally conducted. As the first coating film, a primer is applied in a thickness of from 5 to 10 xcexcm to what will subsequently be the visible side. Following the first pass through the dryer, the actual topcoat is applied. After drying it has a film thickness of approximately 20 xcexcm. In some cases this surface is further laminated, while hot, with a temporary protective sheet. This is intended to protect it against mechanical injury. In parallel with the coating of the visible sides, the reverse sides as well are coated. Primers used include, for example polyester resins. For coil-coated facings and roofs under corrosive industrial atmospheric conditions, epoxy-containing systems are used as primers. As topcoat materials, liquid coating materials in innumerable colors are used primarily. Depending on the field of application, polyester, polyurethane or PVDF topcoat materials, for example, are employed. The film thicknesses of the topcoats are normally about 20 xcexcm.
Besides the liquid primers and topcoats, powder coating materials are also used for the coil coating of metal coils. Powder coating materials have the great advantage over their liquid counterparts of being solvent-free and hence more ecological. However, their proportion among the coil coating systems has to date been relatively low.
One of the reasons was the high powder coating film thickness of more than 40 xcexcm. This leads to optical defects, since the surface is no longer entirely free from pores. This drawback was eliminated by WO 97/47400. It describes a process for coating metal coils, with which powder film thicknesses of less than 20 xcexcm can be obtained.
A second disadvantage as compared with liquid coating materials was the extremely slow coil speed during application of the powder coating material. Using electrostatic spray guns, metal coils can be coated with powder coating material only at line speeds of a maximum of 20 m/min. As a result of the MSC Powder Cloud(trademark) technology, described, for example, by F. D. Graziano, XXIIIrd International Conference in Organic Coatings, Athens, 1997, pages 139-150 or by M. Kretschmer, 6th DFO Conference on Powder Coating Practice, Dresden, 2000, pages 95-100, coil speeds of from 60 to 100 m/min are now realizable.
PU powder coating materials are known, inter alia, for their high weathering stability, excellent leveling, and good flexibility. For use in coil coating, however, the flexibility of the systems known to date is often inadequate. Consequently, the search is on for new PU powder coating materials which satisfy the extreme flexibility requirement of coil coatings. Success in this search, if achieved, would remove the third critical disadvantage relative to conventional liquid coating materials.
EP 0 047 452 describes isocyanato isocyanurates, based on hexamethylene diisocyanate or isophorone diisocyanate, which following blocking of the NCO groups can be used as crosslinkers for producing flexible solvent-borne or pulverulent polyurethane coatings.
EP 0 132 518 describes a composition which is based on polyhydroxy components and trimers of 2-methylpentyl 1,5-diisocyanate, 2-ethylbutane 1,4-diisocyanate, and isophorone diisocyanate and which is a suitable binder for powder coating materials for the coating of heat-curable substrates.
DE 197 29 242 describes pulverulent binders comprising a hydroxyl-containing polyacrylate and physical mixtures of at least one aliphatic isocyanate component containing isocyanurate or urethane or biuret groups and at least one (cyclo)aliphatic isocyanate component containing isocyanurate and/or urethane groups and/or cycloaliphatic isocyanate component containing isocyanurate and/or urethane groups, whose NCO groups are blocked with xcex5-caprolactam.
The powder coating materials described in the aforementioned prior art are used exclusively for coating metal preforms. Their use for coating in accordance with the coil coating processes is not described.
Surprisingly it has been found that a selection of certain crosslinkers from those described above with certain hydroxyl-containing polyesters may be processed to binders which are suitable for coating metallic substrates by the coil coating process.
The invention accordingly provides for the use of polyurethane powder coating materials for coating metal coils by the coil coating process, wherein said materials comprise
A) an isocyanate component which is totally or partly blocked with xcex5-caprolactam and comprises physical mixtures of individual isocyanato isocyanurates of aliphatic and (cyclo)aliphatic and/or cycloaliphatic diisocyanates,
B) polyesters containing hydroxyl groups and terephthalic acid, and
C) if desired, customary auxiliaries and additives, and where there are 0.5-1.2 NCO groups of component A) per OH group of component B).
Thus, the present invention includes a process for coating metal coils, comprising:
coil coating a metal coil with a polyurethane powder coating material which is the reaction product of
A) an isocyanate component which is totally or partly blocked with xcex5-caprolactam and comprises physical mixtures of individual isocyanato isocyanurates of aliphatic and (cyclo)aliphatic and/or cycloaliphatic diisocyanates, and
B) one or more polyesters containing hydroxyl groups and terephthalic acid, and
wherein there are 0.5-1.2 NCO groups of A) per OH group of B).
The invention further provides metal coils coated with polyurethane powder coating materials by the coil coating process, wherein the polyurethane powder coating materials comprise
A) an isocyanate component which is totally or partly blocked with xcex5-caprolactam and comprises physical mixtures of individual isocyanato isocyanurates of aliphatic and (cyclo)aliphatic and/or cycloaliphatic diisocyanates,
B) polyesters containing hydroxyl groups and terephthalic acid, and
C) if desired, customary auxiliaries and additives, and where there are 0.5-1.2 NCO groups of component A) per OH group of component B).
A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description.